Validate input into a Text filed.

I have a text field on a form that requires input. I've tried various code to prevent a blank field but some users have figured out they can type "sdlvgkj" then back space it out and the content is allowed. I've also tried defaulting the field to "You must supply details for this report" but, again, if they highlight it and delete or spacebar it it results in a blank field. Thanks for your help.